1. Field of the Invention
The present invention relates to an image forming apparatus having a mechanism for preventing an optical system from adhesion of dust.
2. Description of the Related Art
There is an image forming apparatus available for exposing a laser beam onto the surface of a photoconductor from a laser beam source, forming an electrostatic latent image on the photoconductor, and forming an image on a sheet of paper by the electrophotographic method.
In such an image forming apparatus, when scanning the laser beam on the surface of the photoconductor, the optical system must be kept clean, though when in use, in the neighborhood of a laser beam emitting window of a laser optical unit (LSU), toner, developer, and dust which are floating in the machine body may be adhered.
If toner, developer, and dust which are floating in the machine body are adhered to the laser beam emitting window of the LSU during writing a latent image on the photoconductor using the laser optical unit, its areas on the photoconductor are not exposed to light, so that in the reproduced image, a line (generally a white line) is recorded perpendicularly to the scanning direction of the laser beam.
Conventionally, to prevent floating toner, developer, and dust from adhering to the laser beam emitting window, several arts are adopted.
For example, a fan is installed, and an air current is generated in the machine body, thus floating toner, developer, and dust are prevented from adhering to the laser beam outgoing window of the LSU.
Further, a transparent negative bias electrode is installed on the laser beam emitting window itself, and at the opposite position, a positive bias electrode having a formed opening in the optical path of the laser beam is installed, thus floating toner is prevented from adhering to the laser beam emitting window (for example, Japanese Patent Application 9-43940).
Further, in the laser beam emitting window of an optical housing for storing the optical system, the shutter which can open or close is installed, and when the printing operation is not in execution, the shutter is closed, thus toner, developer, and dust are prevented from adhering to the laser beam emitting window (for example, Japanese Patent Application 11-167080).
In the prior art 1, by the air current in the machine body which is generated by the fan, toner, developer, and dust floating in the machine body are prevented from approaching the laser beam emitting window, though it is difficult to generate an ideal air current in the machine body, and depending on the internal shape of the machine body, a place where the air current swirls is generated, and toner, developer, and dust floating in the machine body are inversely called, thus it is difficult to prevent them from adhering to the laser beam emitting window.
In the prior art 2, the electrode is installed on the laser beam emitting window itself, so that although it is transparent, due to the variation of the electrode thickness, the adhering condition and the transparency, the optical characteristics such as brightness and refractive index are changed, and the changes adversely affect the laser beam passing, and a problem arises that the effect appears in images.
Further, in the prior art 3, when the shutter is closed, toner, developer, and dust floating in the machine body can be prevented from adhering to the laser beam emitting window, though during the printing operation, the shutter must be opened, and particularly during continuous printing, the shutter is kept opened, and at this time, toner, developer, and dust floating in the machine body cannot be prevented from adhering to the laser beam emitting window.